In a wideband wireless communications system, a communication signal may be decreased by frequency selective fading due to multi-path transmissions. Conventional OFDM systems have attempted to overcome the problem of frequency selective fading by dividing total signal bandwidth into a plurality of sub-carriers, such that bandwidth on each sub-carrier is sufficiently narrow to provide relatively flat fading for data modulation symbols carried by that sub-carrier. Conventional OFDMA systems have used OFDM modulation techniques to multiplex signal traffic, from a plurality of mobile stations, in both frequency and time domains.
Typically, within a conventional OFDM or OFDMA based wireless communications system, a frame structure may comprise a plurality of “superframes”—wherein each superframe may comprise a superframe preamble frame, and a plurality of traffic frames. Each superframe preamble frame, and each traffic frame, may comprise one or more OFDM symbols.
Each OFDM symbol may comprise an inverse fast Fourier transform (IFFT) symbol, which is the result of an IFFT operation on a modulation data sequence. The OFDM symbol may also comprise a cyclic prefix (CP), which is typically a repetition of the last portion of the associated IFFT symbol, and is typically inserted before the IFFT symbol. The OFDM symbol may also comprise windowing sections, to shape the modulation pulse such that the radio spectrum of the transmitted signal meets emission mask requirement set forth by a radio regulatory body (e.g., the Federal Communication Commission (FCC) in the United States).
A cyclic prefix may be added to each IFFT symbol to address problems of inter-symbol interference (ISI) and inter-carrier interference (ICI). In an OFDM or OFDMA based communication system—assuming maximum delay spread of a channel has a known length L—if cyclic prefix length is chosen to be longer than L, ISI and ICI may be avoided completely, and orthogonality between frequency sub-carriers may be maintained. At a receiver, however, the cyclic prefix goes unused and is simply discarded. As a result, although the cyclic prefix helps eliminate ISI and ICI, it nonetheless adds unutilized overhead and reduces overall system efficiency.
In certain instances, cell size and delay spread of a channel may be different between different operational areas. For example, in urban and rural area systems, different cyclic prefix lengths may be used in each to improve the efficiency for each system. Usually, conventional systems utilize varying cyclic prefix length for traffic frames—while using fixed cyclic prefix length for superframe preamble frames—so that there is no ambiguity at a mobile receiver as to which cyclic prefix length to use when decoding superframe preamble frames. Cyclic prefix length information for traffic frames is indicated by a base station using the superframe preamble.
Although some conventional systems have provided variability of cyclic prefix length for OFDM symbols in a superframe preamble in an attempt to improve system efficiency, increased ambiguity of timing for detecting each OFDM symbol in a superframe preamble has heretofore resulted from extended variability or flexibility of cyclic prefix length in conventional systems. This can potentially degrade performance of a superframe preamble, and reduce coverage of a sector.
These considerations must be balanced against fixed cyclic prefix length approaches—where a cyclic prefix length is one default value for all radio environments—since fixed values tend to be excessive for at least some of the radio environments. Excessive cyclic prefix on OFDM symbols in a superframe preamble tend to reduce overall system efficiency.
As a result, there is a need for methods and/or constructs that provide flexible cyclic prefix length on a superframe preamble while maintaining optimal system performance.